1. Field of the Invention
The present invention relates to an arc start control method in robot welding for improved arc start in the retract arc starting in which the welding wire is first shorted with the base metal and then the welding torch is retracted by the welding robot to move the welding wire off the base metal to generate an arc.
2. Description of the Related Art
FIG. 5 is a configuration diagram of a robot welder which performs consumable electrode arc welding. A robot controller RC outputs an operation control signal Mc for controlling operation of a multi-axis servo motor disposed in a robot main body (manipulator) RM, as well as sending and receiving interface signals which include a welding start signal St, a feeding rate setting signal Fr, a welding voltage setting signal Vr and a short/arc determination signal Sa, to and from a welding power source PS. The welding power source PS, which receives and sends the interface signals, outputs a welding voltage Vw and a welding current Iw, as well as outputting a feeding control signal Fc for controlling a wire feeding motor WM. The robot main body RM is mounted with the wire feeding motor WM, a welding torch 4 and so on, and moves a tip (TCP) of the welding torch 4 along a predefined welding line. When the welding wire 1 is fed by the wire feeding motor WM through the welding torch 4, an arc 3 is generated between the wire and the base metal 2, and welding starts.
FIG. 6 is a timing chart of a retract art starting method which uses the above-described robot welder. In the figure, (A), (B), (C), (D), (E), (F), and (G) show the time course change of the welding start signal St, the feeding rate setting signal Fr, the welding voltage Vw, the short/arc determination signal Sa, the welding current Iw, a distance Lt between a power supply chip 4a and a base metal, a distance Lw between the wire tip and the base metal (the distance is an arc length La when the arc is present), respectively. (H1) through (H5) illustrate states at the arc generation location at different time points. Hereinafter, description will be made with reference to FIG. 6.
(1) Time Point t1 through t2; Slow-Down Feeding Period
At Time Point t1, the robot brings and stops the welding torch onto the welding start location. When the robot controller RC outputs the welding start signal St (High level) as shown by (A), the feeding rate setting signal Fr switches for a slow-down feeding rate as shown by (B) and the welding wire is fed at the slow-down feeding rate. Simultaneously, as shown by (C), the welding power source PS starts outputting the power, and thus the welding voltage Vw becomes a no-load voltage. During this period, as shown by (F), the welding torch is at a stop and thus the chip-to-base-metal distance Lt has a constant value. On the other hand, as shown by (G), the wire-tip-to-base-metal distance Lw decreases gradually due to the slow-down feeding.
(2) Time Point t2 through t3; Short-Circuit Period
At Time Point t2, as shown by (H2) in the figure, the wire tip is shorted with the base metal. The welding voltage Vw goes down to assume a low short voltage value as shown by (C), and then this voltage change is detected to turn the short/arc determination signal Sa to the High level (Short) as shown by (D). In response, the feeding rate setting signal Fr becomes zero as shown by (B) to stop the feeding. Simultaneously, as shown by (E), a short-circuit current flows which has a small current value of 20 through 80 amperes approximately. Further, simultaneously, the welding torch starts to move off the base metal (retracting movement) and thus, as shown by (F), the chip-to-base-metal distance Lt increases gradually.
(3) Time Point t3 through t4; Retracting Period
At Time Point t3, as shown by (H3), the retracting movement of the welding torch creates a gap between the wire tip and the base metal, whereby an initial arc 3a is generated. The welding voltage Vw increases to become an arc voltage Va which assumes a few tens of volts as shown by (C), and the short/arc determination signal Sa switches to Low level (Arc) as shown by (D). In response to this, as shown by (E), the welding current Iw switches to an initial arc current Is which is more or less the same level as the short-circuit current. Generally, a constant-current control is provided in order to maintain the initial arc current Is at a consistent value. As shown by (F), the retracting movement of the welding torch continues until a predetermined position is reached at Time Point t4. As shown by (H4), the initial arc 3a is maintained during this period, while as shown by (G), the arc length La (=Lw) increases gradually by a distance equal to the distance corresponding to the retracting movement. This period continues for 100 through 500 ms approximately.
(4) Time Point t4 and thereafter; Steady Period
At Time Point t4, as shown by (F), the welding torch reaches the predetermined position at the end of its retracting movement. Then, the feeding rate setting signal Fr switches for a steady feeding rate as shown by (B), whereby steady feeding of the welding wire is started. Simultaneously, as shown by (E), supply of a steady welding current Ic in consideration of the steady feeding is started. The welding torch starts to move along the welding line. In other words, after Time Point t4, a steady arc start control for consumable electrode arc welding is provided. The arc length La in this period converge to the steady arc length as shown by (G), and the initial arc 3a makes the shift to a steady arc 3b as shown by (H5).
The above description for the period from Time Point t1 to t2 was for a case in which the feeding of the welding wire is made to create short circuit. However, the feeding may be stopped and the welding torch may be moved closer to the base metal (forward movement). The retraction arc starting as described above ensures good arc starting to difficult materials (e.g. aluminum alloys and stainless steels) which would not allow a good arc start under a common method of arc starting. Further, the retraction arc starting enables to dramatically reduce the amount of spatters when welding steel. Due to these characteristics, the retraction arc starting is used in high quality welding. See JP-A 2002-178145 Gazette and JP-A 2002-205169 Gazette for more details of the above-described prior art.
As shown in FIG. 6, at Time Point t3, the initial arc is generated in a small gap created by the retracting movement of the welding torch between the wire tip and the base metal. The retracting movement of the welding torch is continued also thereafter and the arc length increases gradually as shown by (G). Meanwhile, the welding wire is fed through a cable of the welding torch, and during the feeding the wire meanders to some extent in the cable. The amount of meandering represents the amount of play in the feeding. The amount of play in the feeding can increase depending on the welding attitude of the welding torch. Further, as the robot moves, there is always small vibration, and the welding torch also vibrates during the retracting movement. Right after Time Point t3, there is already a small gap at the wire tip, and the above-described play for the welding wire sometimes causes an abrupt change of the wire tip position. Similarly, the vibration of the welding torch also causes an abrupt change in the wire tip position. These changes of the wire tip position are small, being as small as 1 to 2 mm. However, the gap (arc length) right after Time Point t3 is smaller than 1 mm approximately and therefore, if the wire tip moves toward the base metal, the wire tip and the base metal short again. On the other hand, if the wire tip moves away from the base metal, the arc length will become slightly longer, and this does not cause any serious problem. The re-shorting does, however; since the initial arc has already made a molten on the wire tip surface, the re-shorting will cause the wire tip to adhere to the base metal. When this happens, a large electric current of a few hundreds of amperes is necessary in order to break the adhesion, resulting in poor arc start with a large amount of spatters. Therefore, preventing the re-shorting right after the initial arcing is important in order to provide good arc starting.